The invention is directed to an atomic resonance filter for optical receivers operating in the presence of solar background radiation, and are not limited in their field of view (FOV).
Filters of this type are used, as for example and not by limitation, for communications from a first location such as, for example, an airplane or satellite to a second location such as, for example a submarine.
The present state of the art atomic resonance filter used in this manner is a filter pack generally referred to as a pancake filter assembly. As can be seen in the prior art showing in drawing FIG. 1, a prior art filter A is depicted. A central positioned cesium cell B is covered at each open end surface by a first sapphire window C and a second sapphire window D. A heater element E surrounds the outer surface of the cesium cell C for elevating the temperature of the cesium cell at its center to approximately 100 degrees C. by heat transfer through the sapphire windows. The first sapphire window C is covered with a light receiving blue filter F for passing only blue light and the second sapphire window D is covered by an infra-red filter G for passing only infra-red light. A plurality of photo-multiplier tubes H in a number sufficient to substantially cover the light emitting surface of the infra-red filter G, nine are shown. More or less than nine may be used depending on the surface area of the infra-red filter. The output signals from the photo-multiplier tubes are parallel fed to provide an output signal related to communication information carried by the blue light.
In operation, only blue light is passed through the blue light filter F into the cesium cell. The received blue light signal contains communication information and is void of any other light frequency. The blue light signal passes through the first sapphire window and into the heated cesium cell which produces infra-red photons therefrom. The infra-red photons pass through the second sapphire window, infra-red filter, are received by the plurality of photo multiplier tubes and converted to usable electric signals from which the communication information carried by the blue light is extracted.
The typical prior art atomic resonance filter has several undesirable characteristics, namely, they require a large bulky cesium cell; large expensive sapphire windows; they are inefficient as they miss pulses generated by the Titanium fluorescence in the sapphire windows and have no enhanced infra-red light collection; they require high temperatures and high power for operation; a plurality of photo-multiplier tubes are required and because of the large infra-red signal receiving area avalanche photo diodes cannot be utilized for infra-red light to electrical signal conversion; and in addition, the devices are complex, economically costly and require a large volume of space for implementation.
There has not been a simple, inexpensive, compact and improved efficient atomic resonance filter until the emergence of the president invention.